Solving Friction & Scaling Problem: Advice for Dave

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  • Thread starter Thread starter Dave Stroud
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    Friction Scaling
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Discussion Overview

The discussion revolves around estimating the speed of an object moving through water before it begins to plough through mud on the sea floor. Participants explore the relationship between the object's speed in the mud, the friction involved, and the influence of water currents. The scope includes conceptual reasoning and exploratory calculations related to friction and drag forces.

Discussion Character

  • Exploratory
  • Technical explanation
  • Conceptual clarification
  • Debate/contested

Main Points Raised

  • Dave seeks advice on estimating the speed of an object moving through water based on its known speed in mud (0.02 m/s) and the friction involved.
  • One participant questions the influence of the towing force and whether it remains constant in both scenarios, suggesting that the power required to tow the object through mud would differ from that in water.
  • Another participant notes that the current speed of the water remains constant, but the friction from the mud slows the object down, raising the possibility of using friction calculations to estimate the water speed.
  • A participant expresses skepticism about the feasibility of the estimation, highlighting that the object’s constant speed in mud does not provide enough information to determine the water speed due to potential variations in drag.
  • One participant suggests that calculating the shear strength of the mud might help in understanding energy loss and its relation to the object's speed prior to ploughing.
  • Another participant explains that if the object moves at a constant velocity, the drag force from the mud and the force from the water must balance, indicating that calculating the drag force could lead to estimating the water flow speed.
  • Dave later mentions that investigations into the geotechnics of the mud indicate that the friction compared to the size of the object would be minimal.

Areas of Agreement / Disagreement

Participants express differing views on the feasibility of estimating the water speed based on the object's behavior in mud. There is no consensus on whether the calculations proposed can yield a reliable estimate.

Contextual Notes

Participants acknowledge limitations in their understanding of hydrodynamics and the relationship between shear forces and drag forces, which may affect the accuracy of any proposed calculations.

Dave Stroud
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I am looking for some advice. I am not a physicist but I have a problem that I am unsure of how to solve.

This is the scenario; I have an object that is ploughing through mud on the sea floor at a constant speed of 0.02 m per second. Prior to the contact with the sea mud it was freely moving at a constant speed in the water. What I would like to do is calculate this speed when it was freely moving in the water. Essentially I would like to create some sort of scaling figure that uses the friction with the mud and the speed it is traveling through it to estimate a constant speed prior to ploughing.

This just has to be a rough sort of estimate, nothing precise. The shape of the ploughing object is a parallelepiped. Please excuse my lack of mechanics knowledge but I am just looking to see if this is actually possible. Any help would be greatly appreciated.

Many thanks
Dave
 
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I doubt I can help with a complete solution but...

Something must be pushing/pulling the object through the mud? Is that the same in both cases? For example if the object was being towed and the tension in the tow rope was say 1000N then the power needed to tow it through the mud is 1000 * 0.02 = 20 Watts. Presumably the towing device will find it easier to tow the object in water but how fast it goes depends on how the towing device works. eg would the towing device try to maintain the same velocity? or would it allow the object to speed up until the power used is the same? or ??
 
Thanks for getting back to me. It is basically the water current that is moving the object. This object is moving with the current before some change in the sea floor means it is shallower and the object begins to then plough the mud. Current speed remains constant throughout but once it begins to plough the mud the friction slows the object down. I know the speed of the object as it moved through the mud (0.02) so I was wondering if it would be possible to use some sort of friction calculation that would take account of this with the speed and thus allow a rough estimate of the current to be calculated? I hope that makes sense?
 
Unfortunately I'm pretty sure it's not possible.

If it moves at a constant 0.02m/s in the mud it must still be influenced by the current otherwise it would slow down. However you have no way of knowing how fast the water is going. eg You can't tell the difference between...

a) The mud is causing very little drag and the water is going at say 0.04 M/S
b) The mud is causing a lot of drag and the water is going at 0.4 M/S.
 
Ah that makes a lot of sense. I am assuming that the energy loss of the object is a function of the shear characteristics and most likely the shear strength of the mud. By calculating the shear strength of the mud do you think it would be possible to calculate how much energy was lost and use that to inform about the speed prior to ploughing?
 
If it's going at constant velocity (eg not accelerating) the drag force due to the mud and the force due to the flow of water must be equal magnitude but opposite directions (eg they sum to zero). So if you can somehow calculate the drag force due to the mud you can work out the equal force on the object due to the flow of water.

Then if you have info on the drag properties of the object in water you could in principle take a stab at calculating the velocity of the water flowing past the object.

I'm afraid hydrodynamics really isn't my field. I'm an electronics engineer. I don't know how (or even if it's possible) to calculate drag forces from shear forces.
 
Many thanks for your advice. Some investigations into the geotechnics of the mud has shown that the friction when compared to the size of the object would be minimal.
 

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